An exhaust system conducts hot exhaust gases generated by an engine through various exhaust components to reduce emissions. An engine coolant circuit is utilized by the engine to prevent the engine from overheating. Roughly one third of engine energy is lost in exhaust and another one third is lost in engine coolant. Some of this exhaust energy is recovered using a Rankine cycle which uses a working fluid to convert heat to work. The working fluid in a Rankine cycle follows a closed loop and is reused constantly. Typically, the working fluid is water, which is pumped from low to high pressure at the beginning of the cycle. The high pressure liquid is then heated by engine exhaust gases to become a vapor. The vapor enters a turbine to generate a work output, such as electrical energy for example. Condensation of the fluid in the turbine occurs such that fluid exiting the turbine is a low pressure vapor, which is subsequently fed into a condenser and then returned to a pump at the beginning of the cycle.
As discussed above, exhaust energy is recovered through the Rankine cycle. However, engine coolant heat is identified as a “low grade” source and is not considered as being capable of serving as a source for energy recovery. Recovering energy from the coolant in addition to recovering the exhaust energy would significantly improve engine efficiency.